An electronic apparatus used in a vehicle control system such as an anti-lock braking system (ABS), or an electronic stability control (ESC) needs both a power electronic device for driving an actuator and a control electronic device for controlling the actuator. Conventionally, the power and control electronic devices are mounted together on a multilayer printed circuit board to form an electronic circuit board as a major component of the electronic apparatus.
As shown in FIG. 9, a conventional electronic circuit board 110 includes power electronic devices P1-P3 and control electronic devices S1-S3. The power electronic devices P1-P3 and control electronic devices S1-S3 are mounted together on a printed circuit board 9. The power electronic devices P1-P3 are a solenoid relay, a power zener diode, an aluminum electrolytic capacitor, a motor relay, or the like. Each of the power electronic devices P1-P3 operates with a large current to drive the actuator. The control electronic devices S1-S3 are an integrated-circuit (IC), a microcomputer, a solenoid driver, or the like. Each of the power electronic devices S1-S3 operates with a small current to control the actuator. The printed circuit board 9 is a glass epoxy board having multiple wiring layers.
All necessary circuits are mounted on the printed circuit board 9. Therefore, the electronic circuit board 110 can be easily assembled with a case into the electronic apparatus.
Since the power electronic devices P1-P3 operate with the large current, each of the wiring layers of the printed circuit board 9 needs to resist the large current. Further, the power electronic devices P1-P3 generate heat. Therefore, the wiring layers need to not only resist the large current but also release the heat.
However, the wiring layers are made from a relatively thin metallic sintered-paste or film. Therefore, the printed circuit board 9 needs to have a large size to allow the thin wiring layers to resist the large current and release the heat. As a result, the electronic apparatus is increased in size.
As shown in FIG. 10, another conventional electronic circuit board 120 disclosed in JP-2004-200464A includes a busbar 10 having wiring portions 11 and terminal portions 12. Electronic devices 30, 31 are mounted on the wiring portions 11 through solder portions 40. The wiring portions 11 except the solder portions 40 are coated with a solder resist 60.
The busbar 10 is housed in a case 20 made of an electrical insulating material such as ceramics or resin. The case 20 consists of an upper case 21 and a lower case 22. The wiring portions 11 are sandwiched between the upper and lower cases 21, 22 and the upper and lower cases 21, 22 are joined together by means of mechanical engagement or adhesive engagement. Thus, the wiring portions 11 are joined together and housed in the case 20. The terminal portions 12 project outwardly from the case 20.
The busbar 10 is a metal plate greater in thickness than the wiring layers of the electronic circuit board 110 of FIG. 9. Therefore, the busbar 10 can resist the large current for operating the electronic devices 30, 31 and release the heat generated by the electronic devices 30, 31 sufficiently.
Typically, when the busbar is used as the wiring circuit, through hole devices (THD) are used as the electronic devices. The busbar except the terminal portions is molded with the resin case and lead wires of the THDs are connected to the terminal portions by laser welding.
In contrast, in the electronic circuit board 120, surface mount devices (SMD) are used as the electronic devices and the SMDs are soldered on the busbar 10 at once in a thermal reflow process. Therefore, high-density mounting is available and the electronic circuit board 120 can be manufactured at low cost as compared to the electronic circuit board having the THDs mounted by laser welding.
In the electronic circuit board 120, however, the wiring portions 11 are sandwiched between the upper and lower cases 21, 22 to be joined together. This approach limits flexibility of circuit design. Therefore, the electronic circuit board 120 cannot achieve a large-scale circuit for the electronic apparatus used in the ABS or ESC.